Holographic recording medium and method for manufacturing the same

ABSTRACT

A holographic recording medium which has parallelism of high precision and causes little substrate deformation ascribable to contraction of its recording areas when recording interference fringes, and a method for manufacturing the same. The holographic recording medium has a holographic recording material layer between a first transparent substrate and a second transparent substrate. This holographic recording material layer is formed integrally with a spacer which is composed of a large number of spherical beads arranged so as to surround recording areas for interference fringes to be recorded on.

TECHNICAL FIELD

The present invention relates to a holographic recording medium and amethod for manufacturing the same.

BACKGROUND ART

Conventional holographic recording media are formed by sandwiching adried liquid holographic recording material, called as a photopolymermaterial, between a pair of transparent substrates.

A manufacturing method thereof includes: forming a frame on the top of atransparent substrate; injecting a photopolymer material into the frame(when any solvent is used, dry until most of the solvent in the materialevaporates) for jellification; placing the transparent substrate on alower press stage of a press machine with the material upward; pressinga second transparent substrate from the side of an upper press stage;and irradiating the outer periphery with ultraviolet rays in thispressed state, thereby curing the periphery of the holographic recordingmaterial before taken out.

During the foregoing pressing, the upper press stage and the lower pressstage must be fine adjusted to a parallel space having a predeterminedthickness (for example, 100 μm or so).

For this reason, conventional measures have been taken such that thetransparent substrates are irradiated with light and the interferencefringes of the reflected light are checked to increase the precision ofthe parallelism of the press stages for fabrication. Otherwise, spacershaving the intended thickness of the holographic recording medium,including the transparent substrates, are used for pressing.

For the sake of increasing the precision of the parallelism of the pressstages or using spacers having the same thickness as that of theholographic recording medium as described above, it takes time tofabricate the holographic recording medium. Improving the precision ofthe parallelism of the press stages also has limitations. Furthermore,when interference fringes are recorded onto the holographic recordingmedium, polymerization contraction can occur in the holographicrecording material to cause deformation of the transparent substrates.In this case, if the contraction before and after holographic recordingincreases, there arises a problem because it becomes difficult toreproduce the recorded information accurately.

DISCLOSURE OF THE INVENTION

This invention has been achieved in view of the foregoing problems. Itis thus an object of the invention to provide a holographic recordingmedium and a method for manufacturing the same which improve theprecision of the parallelism with a simple configuration and allowmanufacturing in a short time.

Another object is to provide a holographic recording medium and a methodfor manufacturing the same which suppress deformation of the transparentsubstrates ascribable to polymerization contraction of the holographicrecording material during holographic recording.

The present inventor has made intensive studies and found that ifrecording areas of the holographic recording material layer aresurrounded with beads or fibers, which also functions as a spacerbetween the pair of transparent substrates, then the precision of theoverall parallelism is improved, the manufacturing is facilitated, andthe deformation of the transparent substrates ascribable to thepolymerization contraction of the holographic recording material duringholographic recording can be avoided.

In summary, the above-described objectives are achieved by the followingembodiments of the present invention.

(1) A holographic recording medium comprising: two transparentsubstrates; a holographic recording material layer sandwichedtherebetween; and a spacer integrally embedded in this holographicrecording material layer, the spacer being composed of at least either alarge number of beads or fibers for regulating a gap between the twotransparent substrates, wherein the spacer is arranged around arecording area of the holographic recording material layer.

(2) The holographic recording medium according to (1), wherein thespacer is formed in a continuous lattice configuration, and therecording area is formed in each lattice cell.

(3) The holographic recording medium according to (1) or (2), whereinthe spacer is composed of a large number of spherical beads.

(4) The holographic recording medium according to (1) or (2), whereinthe spacer is composed of a plurality of fibers, and the fibers form atleast one connection gap therebetween for each of the recording areas.

(5) The holographic recording medium according to (1) or (2), whereinthe spacer is composed of fibers, and necked parts for letting a liquidholographic recording material in and out of the recording area areformed in peripheries of the fibers in a longitudinally intermittentfashion.

(6) A method for manufacturing a holographic recording medium,comprising: a step of forming a frame for surrounding at least onerecording area on a transparent substrate; a step of injecting a liquidholographic recording material into the frame; a step of arranging aspacer composed of at least either a large number of beads or fibersalong the frame before detaching the frame from the holographicrecording material; a step of attaching the transparent substrate to onepress stage with a layer of the holographic recording material upward; apress step of pressing a second transparent substrate against the layerof the holographic recording medium by using another press stage via anelastic member; and a step of curing at least periphery of the layer ofthe holographic recording material in this pressed state.

(7) The method for manufacturing a holographic recording mediumaccording to (6), comprising the step of arranging another spacerbetween the spacers arranged along the frame, thereby defining aplurality of recording areas in an area surrounded by the spacersarranged along the frame.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged cross-sectional view showing a holographicrecording medium according to a first embodiment of the presentinvention.

FIG. 2 is a cross-sectional view taken along the line II-II of FIG. 1.

FIG. 3 is a cross-sectional view similar to FIG. 2, showing aholographic recording medium according to a second embodiment of thepresent invention.

FIG. 4 is a cross-sectional view of a portion corresponding to the lineIV-IV of FIG. 3.

FIG. 5 is a cross-sectional view similar to FIG. 2, showing aholographic recording medium according to a third embodiment of thepresent invention.

FIG. 6 is a cross-sectional view of a portion corresponding to the lineVI-VI of FIG. 5.

FIG. 7 is a cross-sectional view similar to FIG. 2, showing aholographic recording medium according to a fourth embodiment of thepresent invention.

FIGS. 8(A)-8(E) are schematic diagrams showing a method formanufacturing a holographic recording medium according to an embodimentof the present invention.

FIG. 9 is a flowchart showing the manufacturing method.

FIG. 10 is a front view showing a press machine to be used in themanufacturing method.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention achieves the foregoing objects by arranging beadsor fibers having an identical diameter as a spacer in a holographicrecording material layer so as to surround recording areas.

FIRST EMBODIMENT

Hereinafter, a first embodiment of the present invention will bedescribed with reference to FIG. 1.

A holographic recording medium 10 of the first embodiment is formed bysandwiching a holographic recording material layer 18, containing aspacer 16, between a first transparent substrate 12 and a secondtransparent substrate 14. The spacer 16 creates a gap between the firstand second transparent substrates 12 and 14 to improve the parallelism.Moreover, the spacer 16 is made of a large number of glass, zirconia, orother spherical beads 16A having an identical diameter. The space 16 isembedded in the holographic recording material layer 18 between thefirst and second transparent substrates 12 and 14, and is arranged in alattice configuration as shown in FIG. 2 so that the holographicrecording material layer 18 inside the cells makes recording areas 20.

Each single recording area 20 is configured to accommodate one or aplurality of spots of a recording laser beam. Provided that therecording laser beam has a spot diameter (an area where an object beamand a reference beam interfering in the holographic recording materiallayer 18 produce interference fringes, i.e., where information isrecorded) of 1 to 2 mm, the recording areas 20 are thus made at leastgreater than a circle of 1 to 2 mm in diameter. It should be appreciatedthat considering polymerization contraction of the holographic recordingmaterial in the recording areas 20 during recording, the spots in therecording areas 20 are preferably fewer.

In the holographic recording medium 10, the overall parallelism iscreated by the spherical beads 16A independent of the precision of thepress machine. Variations in the diameter of the spherical beads 16A canthus be reduced to improve the parallelism and make the gap between thefirst and second transparent substrates 12 and 14, i.e., the thicknessof the holographic recording material layer 18 uniform as well.

The diameter of the spherical beads 16A is selected in accordance withthe intended thickness of the holographic recording material layer 18.Here, it shall be 0.1 to 2.0 mm.

When interference fringes are recorded onto the recording areas 20, theholographic recording material causes polymerization contraction. Sincea tension resulting from this polymerization contraction is cut off bythe spacer 16 which is sandwiched and fixed by the first and secondtransparent substrates 12 and 14, it is possible to suppress deformation(warp) of these first and second transparent substrates 12 and 14.

Here, the first and second transparent substrates 12 and 14 are made ofa material that is optically flat and transparent to the light sourcewavelengths for use in holographic recording and reproduction, such asoptical glass materials including BK7 and synthetic quartz, and plasticmaterials including polycarbonates and PMMA.

SECOND EMBODIMENT

Next, description will be given of a holographic recording medium 30according to an embodiment of the present invention shown in FIGS. 3 and4.

This holographic recording medium 30 is obtained by replacing the spacer16 in the holographic recording medium 10 of the first embodiment shownin FIG. 1 with a spacer 32 which is composed of a large number of fibers32A having the same diameter. Like the spherical beads 16A, these fibers32A have a diameter of 0.1 to 2.0 mm, for example.

Since the rest of the configuration is the same as in the firstembodiment, like parts will be designated with identical numerals tothose given to the components shown in FIG. 1 or 2. Description thereofwill thus be omitted.

In the holographic recording medium 30 according to this secondembodiment, the spacer 32 is formed by arranging the fibers 32A, such ashard resin fibers and glass fibers cut into appropriate lengths, in alattice configuration.

With the foregoing spacer 32, the individual fibers 32A create slightgaps 33 therebetween so that a liquid holographic recording materialduring manufacturing (to be detailed later) can flow into/out of thelattice cells.

In this second embodiment, the use of the fibers 32A facilitates thearranging operation during manufacturing as compared to the case wherethe spherical beads 16A are arranged in a lattice configuration in theforegoing first embodiment.

Besides, the fibers 32A have higher resistance than that of thespherical beads to the polymerization contraction when interferencefringes are recorded onto the recording areas 20, and can thus bettersuppress deformation of the first and second transparent substrates 12and 14.

THIRD EMBODIMENT

Next, description will be given of a third embodiment shown in FIGS. 5and 6.

In the holographic recording medium 30 of the foregoing secondembodiment, the spacer 32 is formed by arranging the fibers 32A in alattice configuration, in which case the individual fibers 32A creategaps 33 therebetween to secure the fluidity of the liquid holographicrecording material during manufacturing. With a spacer 42 of aholographic recording medium according to the third embodiment, however,fibers 42A are arranged closely and appropriate ones of the fibers 42Aare provided with necked parts 43 so as to secure the inflow and outflowof the liquid holographic recording medium.

In this third embodiment, the fibers 42A can be arranged with no spacetherebetween, which facilitates the arrangement during manufacturing.Moreover, since the fibers 42A are arranged in contact with one another,they have a high resistance to the polymerization contraction thatoccurs when interference fringes are recorded onto the recording areas20. Deformation of the first and second transparent substrates 12 and 14can thus be suppressed further.

FOURTH EMBODIMENT

Next, with reference to FIG. 7, description will be given of aholographic recording medium 50 according to a fourth embodiment of thepresent invention.

This holographic recording medium 50 has a spacer 52 which is made of amixture of spherical beads 52A and fibers 52B.

Take, for example, the case where the recording areas 20 are somewhatmade slightly greater than the diameter of the spherical beads 52A asshown in FIG. 7, i.e., that a single recording area is irradiated with asingle (laser beam) spot. Then, the use of the fibers 52B for thehorizontal direction of the diagram and the spherical beads 52A for thevertical direction eliminates the need to use short cut fibers, andfacilitates the configuration.

FIFTH EMBODIMENT

Now, the process of manufacturing of the foregoing holographic recordingmedium shown in FIGS. 8(A)-8(E) will be described with reference to aflowchart of FIG. 9.

Initially, at step 101, a frame 54 is formed on the first transparentsubstrate 12 as shown in FIG. 8(A). Next, proceeding to step 102, aliquid holographic recording material, or hybrid material 56, isinjected into the frame 54.

Here, examples of the hybrid material 56 include an inorganic glassnetwork filled with a photopolymer, such as one described in JapanesePatent No. 3039165.

Proceeding to step 103, spherical beads 58 are arranged along the innerperiphery of the frame 54. Here, if more than a certain amount ofsolvent in the hybrid material 56 is evaporated, the spherical beads 58cannot be put into the hybrid material 56 due to increased viscosity ofthe hybrid material 56. It is therefore preferable to arrange thespherical beads 58 immediately after the hybrid material 56 is injectedinto the frame 54 (see FIG. 8(B)). It should be noted that the sphericalbeads 58 shall have a diameter of 100 μm here.

Next, proceeding to step 104, the frame 54 is removed as shown in FIG.8(C). This timing is approximately 30 minutes after the injection of thehybrid material 56 into the frame 54. During this time, some of thesolvent in the hybrid material 56 evaporates to increase the viscosityof the hybrid material 56. This precludes the hybrid material 56 fromflowing out through the gaps of the spherical beads 58 even when theframe 54 is removed.

In this state, the hybrid material 56 is dried for two days, forexample, until most of the solvent inside evaporates. The hybridmaterial 56 is thus jellified.

Next, at step 105, the jellified hybrid material 56 is set into a pressmachine 60 (see FIG. 10 for details) along with the first transparentsubstrate 12 as shown in FIG. 8(D).

Here, the first transparent substrate 12 and the jellified hybridmaterial 56 are placed on a lower press stage 61 of the press machine 60with the hybrid material 56 upward. Meanwhile, the second transparentsubstrate 14 is attached to an upper press stage 62 of the press machine60 via ail elastic member 63. This elastic member 63 is made of hardrubber or the like, for example.

Now, description will be given in detail of the press machine 60 withreference to FIG. 10. The press machine 60 comprises: the lower pressstage 61 which is installed on a table 64; the upper press stage 62which is arranged so as to be movable in up and down directions by meansof guide posts 65 erected on the table 64, and is opposed to the lowerpress stage 61; stopper units 66 which regulate the range of downwardmovement of the upper press stage 62; and the elastic member 63 which isattached to the lower side, in FIG. 10, of the upper press stage 62.

The stopper units 66 comprise: columns 66A which are erected on thetable 64; bolts 66C which are attached to the top ends of these columns66A via spacer parts 66B so as to be capable of adjustment in heightposition; and blocks 66D which are attached to the upper press stage 62and come into contact with the bolts 66C when the upper press stage 62is lowered. The thickness of the spacer parts 66B can be changed to setthe limit of downward movement of the upper press stage 62.

According to this embodiment, the spacer parts 66B of the stopper units66 are given a thickness that is selected so that the second transparentsubstrate 14 supported on the upper press stage 62 via the elasticmember 63 can be pressed against the jellified hybrid material 56 on thelower press stage 61 sufficiently, even in consideration of the elasticdeformation of the elastic member 63.

As shown in FIG. 8(E), the upper press stage 62 is lowered to press thesecond transparent substrate 14 against the first transparent substrate12 and the jellified hybrid material 56. In this state, at step 106, theperiphery of the jellified hybrid material 56 is irradiated withultraviolet rays (UV) for curing.

Next, proceeding to step 107, the upper press stage 62 is lifted afterthe UV curing of the periphery. The holographic recording medium 10composed of the first transparent substrate 12, the hybrid material 56,the spherical beads 58, and the second transparent substrate 14integrated is thus taken out of the press machine 60.

After taken out, the hybrid material 56 having the cured periphery isfurther irradiated with UV at inner areas for curing.

The holographic recording medium manufactured by the manufacturingmethod shown in the foregoing FIGS. 8 and 9 was made, for example, byusing sodium-lime glass substrates (35 mm×75 mm×1.2 t: high precisionsin parallelism and flatness) for the first and second transparentsubstrates 12 and 14. As a result, the holographic recording mediumshowed variations of 5 μm in the total thickness and a parallelism of0.77 μm/cm.

On the contrary, best results obtained from conventional manufacturingmethods, i.e., by adjusting the parallelism between the lower pressstage 61 and the upper press stage 62 of the press machine 60 and fineadjusting the stopper units 66 and the like, even without using theelastic member 66, were total thickness variations of 9 μm and aparallelism of 1.38 μm/cm under the same condition as mentioned above.

It should be noted that, while the foregoing embodiment of themanufacturing method has used spherical beads 58 as the spacer, thismanufacturing method is not limited to spherical beads. Fibers may beused as in the foregoing holographic recording media 30, 40, and 50.

Beside, while the foregoing embodiment of the manufacturing method hasused the liquid hybrid material 56 as the holographic recordingmaterial, the present invention is not limited thereto. Photopolymermaterials (solvent-free type) may also be used. In this case, the dryingprocess at step 104 of the flowchart shown in FIG. 9 becomesunnecessary.

It should be appreciated that in the foregoing embodiments, the diameterof the spacer, i.e., spherical breads or fibers may be determined inaccordance with the thickness of the holographic recording materiallayer. With reference to the target value of the parallelism of theholographic recording medium, beads or fibers may be used selectivelyapplication by application, depending on their materials (glass,zirconia, etc.) and the precision of variations in diameter.

INDUSTRIAL APPLICABILITY

In the holographic recording medium according to the present invention,the overall parallelism does not depend on the precisions of pressmachines. Variations in the diameter of the spacer can thus be reducedto improve the parallelism and make the holographic recording materiallayer uniform in thickness. In addition, since the spacer is formed tosurround the recording areas, it is possible to suppress a warp of thesubstrates ascribable to the polymerization contraction of the recordingmaterial during holographic recording.

1. A method for manufacturing a holographic recording medium,comprising: a step of forming a frame for surrounding at least onerecording area on a transparent substrate; a step of injecting a liquidholographic recording material into the frame; a step of arranging aspacer composed of at least either a large number of beads or fibersalong the frame before detaching the frame from the holographicrecording material; a step of attaching the transparent substrate to onepress stage with a layer of the holographic recording material upward; apress step of pressing a second transparent substrate against the layerof the holographic recording material by using another press stage viaan elastic member; and a step of curing at least periphery of the layerof the holographic recording material in this pressed state.
 2. Themethod for manufacturing a holographic recording medium according toclaim 1, comprising the step of arranging another spacer between thespacers arranged along the frame, thereby defining a plurality ofrecording areas in an area surrounded by the spacers arranged along theframe.